Carbon fibers of the type produced by using fibers of acrylonitrile polymer or an acrylonitrile type polymer as a raw material are widely known and have already been put to practical use. The carbon fibers are generally obtained by preoxidizing the aforementioned raw material in an oxidizing atmosphere such as in air at from about 200.degree. to 300.degree. C., and further carbonizing the preoxidized fibers in an inert gas such as nitrogen, argon, or helium at from about 500.degree. to 1,500.degree. C. Graphite fibers are obtained by graphitizing the carbon fibers in an inert gas such as those described above at a temperature in the range of from about 2,000.degree. to 3,500.degree. C. In such a manner graphite fiber having a modulus of elasticity of from 35,000 to 70,000 kgf/mm.sup.2 can be obtained.
The thus obtained carbon fibers or graphite fibers are combined with various thermosetting resins or thermoplastic resins such as epoxy resin or polyimide resin to produce light but strong composites to be used in the fields of sporting goods, aviation, and the space industry.
Conventional carbon fibers, however, have a tensile strength of 450 kgf/mm.sup.2 at most. The graphite fibers obtained by graphitizing such carbon fibers acquire an increased tensile modulus of elasticity, and notably lose tensile strength to even less than 300 kgf/mm.sup.2, a magnitude much smaller than the strength of carbon fibers. Inevitably it has been customary, therefore, to make a choice between carbon fibers and graphite fibers depending on the nature of use contemplated, as adopting carbon fibers for uses requiring strength, and graphite fibers for uses requiring elasticity.
In recent years, the desirability of supplying carbon fibers and graphite fibers combining still greater strength and still higher elasticity for various uses, such as in the aircraft and space industry, has been finding growing recognition with a view to utilizing improved performance.
It has been ascertained by the inventors through their study that graphite fibers, as compared with carbon fibers yet to be graphitized, are liable to lose adhesiveness with resin as a consequence of the development of the structure of graphite crystals. As means of enhancing the adhesiveness, there may be conceived an idea of oxidizing the surface of graphite fibers, similarly to that of carbon fibers, thereby make the surface of the structure of graphite crystals amorphous to some extent, and, at the same time, enabling the graphite fibers to produce a chemically functional group. In spite of this effort, it is still difficult to make graphite fibers manifest the effects of this surface treatment to advantage. Conversely, the effort brings about the disadvantage that the surface treatment performed using a high energy tends to cause a decline of the strength of fibers.
Further, since graphite fibers are inherently brittle, the bundle of graphite fibers during the course of manufacture inevitably produce fluff heavily. As a solution to this problem, there may be conceived a method of precluding the formation of fluff and the growth of fluff produced by applying a readily decomposable high molecular substance such as, for example, a polyoxyethylene polymer, water-soluble saturated polyester, nonionic surfactant, or butene polymer in advance on the bundle of carbon fibers as a raw material and then feeding the bundle to the step of graphitizing treatment. Unfortunately, this method is not sufficiently effective.
If an epoxy resin, unsaturated polyester, methyl cellulose, or carboxymethyl cellulose, for example, is used for the application on the bundle of carbon fibers with a view to precluding the occurrence of fluff and the growth of fluff formed at all, one encounters the problem that the produced bundle of graphite fibers is surface treated with increased difficulty and suffers from a loss of strength.